This invention relates generally to transmission line receivers and more particularly to such receivers that have a high degree of common-mode rejection that will work in a high input signal voltage environment.
In the transmission of data between two physically separated computer systems the ground potential differential between the systems must be controlled within established limits. Each signal transferred between the machines is sent via a connecting cable comprising a pair of wires driven differentially by a driver circuit. At some distance down the connecting cable a receiver circuit is provided to sense the differential signal, provided by the driver, and to convert it to a single ended signal that will conform to the minimal accepted levels within the receiving system.
Since the driver circuit provides a signal referenced to its local ground and its power supply, the signal it sends to the receiver will appear at the receiver to experience a common-mode shift when the receiver reference, i.e. its own local ground and its own power supply is at a different level. The magnitude of this apparent shift is of course equal to the ground potential difference between the transmitting and the receiving systems. For example, a 7 volt maximum ground shift means that the receiving circuit must be able to sense differential signals in the range of hundreds of millivolts riding on a common-mode signal whose magnitude can be up to 7 volts outside the receiving circuit power supply voltage. Since the receiver circuit is usually provided with a single +5 voltage supply with respect to the local ground the receiver must be able to function over a common-mode range -7 volts to +12 volts with respect to its own local ground.
It is therefore imperative that a receiver circuit be available that has a high degree of common-mode rejection yet will operate over a wide signal voltage environment.